In the furnace of the fluidized bed boiler, the combustion takes place in a so-called fluidized bed consisting of solid particulate bed material which is kept in a fluidized state by means of fluidizing air supplied from underneath. At the same time, fuel is supplied continuously into the furnace to maintain the combustion process. The thermal energy produced by the combustion is transferred primarily to heat transfer surfaces of the walls of the furnace, to heat transfer medium flowing in their tubes, and furthermore, energy is also recovered from flue gases exiting from the furnace.
Underneath, the furnace is limited in the horizontal plane by the grate which comprises elongated elements next to each other, fluidizing air being supplied through the elements into the furnace. The elements may be, for example, so-called box beams. Fluidizing air is supplied into the box beams and distributed into nozzles in the beams, for supplying the fluidizing air evenly over the grate area. Through openings left between the elements, material can be removed from the bed into a discharge unit underneath the grate. Examples of grate structures for a fluidized bed boiler are presented, among others, in U.S. Pat. Nos. 5,743,197 and 5,966,839.
Various types of fuels can be used in fluidized bed combustion. The combustion conditions in the fluidized bed boiler may vary, depending on the fuel. If, for example, the fuel has a high adiabatic combustion temperature, the heat transfer surfaces of the walls of the furnace are not sufficient to keep the temperature of the bed in a suitable range. One approach is to use circulation gas for cooling, but this will reduce the efficiency of the boiler. On the other hand, the bed temperature cannot be allowed to rise too high, because it will easily cause sintering of the bed material.
A known method for cooling the bed to a suitable combustion temperature is to equip the furnace with heat transfer tubes extending through it in the horizontal direction, for example between opposite walls. The tubes can be installed on top of each other to form bundles which can be supported to each other by means of connecting tubes extending crosswise between the bundles. Such heat transfer surfaces “immersed” in the fluidized bed are disclosed e.g. in the German published patent application 3347083. The heat transfer surfaces disclosed in said publication consist of bundles of quadrangular tubes stacked on top of each other, bundles of round tubes stacked on top of each other and equipped with a protective layer, or groups of separate pipes equipped with vertical protective wings. In said publication, the aim is to arrange the side walls of the heat transfer surfaces as vertical as possible so that the bubbling of the fluidized bed and the vertical motion of its material would cause as little erosion as possible in the heat transfer surfaces. Other approaches to protect the heat transfer surfaces from the erosive effects of the fluidized bed and from corrosion are disclosed, for example, in German published patent applications 3431343 and 3828646 as well as in European patent 349765.
Now, the bubbling of the fluidized bed and the movements of the material therein, caused by the fluidizing air, subject any heat transfer surfaces extending across the furnace to erosion. Therefore, in said patents, attempts have been made to minimize the loading of the heat transfer surfaces by arranging the side walls of the heat transfer surfaces as vertical as possible, i.e., parallel to the primary direction of movement of the bed material. In these arrangements, the heat transfer surface structures extend in the horizontal direction across the bed in the inner volume of the furnace. However, the problem is that particularly the lower part of said structures is subjected to the erosive effect of the fluidizing air and the fluidized bed material, and furthermore, the movements of the bed cause vibrations which may reduce the strength of the structures, for example the protective layer of the pipes. In European patent 349765, heat transfer pipes placed on top of each other are protected on both sides by vertical shields, a kind of a housing arrangement, in which a horizontal gap is left at the upper and lower edges of the housing. The gap at the lower edge throttles the flow of air to such an extent that it cannot fluidize the fluidized bed material in the space between the protective shields. However, the lower parts of the protective shields on both sides of the gap remain exposed to the effects of the fluidizing air and the bed material, and furthermore, said structure is subjected to clogging.